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Derivative Instruments (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
MWh
quote
Dec. 31, 2015
USD ($)
Purchase power contract    
Derivative instruments    
Number of broker quotes received to determine fair value of derivative instrument | quote 1  
GSWC    
Derivative instruments    
Derivative Activity Volume | MWh 352,000  
GSWC | Purchase power contract    
Changes in the fair value of the derivative    
Balance, at beginning of the period $ (7,053) $ (3,339)
Unrealized gain (loss) on purchased power contracts 2,152 (3,714)
Balance, at end of the period $ (4,901) $ (7,053)
Minimum | GSWC | Purchase power contract    
Derivative instruments    
Derivative, Term of Contract 3 years  
Maximum [Member] | GSWC | Purchase power contract    
Derivative instruments    
Derivative, Term of Contract 5 years